Modern radio communication systems are developed to be able to support various kinds of network services to end users. Apart from ordinary speech connections such services can relate to, e.g., the transfer of video data or the download of portions of packet data. Apparently, this requires a radio communication network that is prepared to handle communication connections for carrying traffic with considerably different characteristics. Said traffic can be distinguished, e.g. by means of the required transmission properties, e.g. bandwidth, delays, or bit error rates; the main traffic direction, i.e. bidirectional or mainly asymmetric; or the traffic density, i.e. whether the traffic flow is more or less constant or occurs in bursts. Four types of traffic classes have been defined for the UTRAN environment with respect to requirements on, e.g., delay, delay variations, or packet loss: The traffic class “conversational” requires typically short delays, a minimum of delay variation, and in-sequence delivery but a moderate packet loss. Examples of applications are bi-directional person-to-person communication such as voice or video telephony and games. Another traffic class, “streaming”, requires moderate delays, a minimum of delay variations, in-sequence delivery but low to moderate packet loss. Examples of applications are unidirectional communication to humans, e.g. real-time audio and video streaming. A third traffic class “interactive” denotes typically information transfer from a server to a human or computer, e.g. Internet-related exchange of information, and requires reasonable low delay and low packet loss. Finally, a fourth traffic class denotes “background” traffic that relates to, e.g., the transfer of e-mail, files, or short messages and requires low packet loss but used for services that can accept longer delay and high delay variations.
In third generation cellular mobile communication systems, e.g. WCDMA-based communication systems, a number of radio channels with different characteristics have been defined in order to be able to handle traffic that is characterised by such various requirements:
A common channel is common to a number of user equipments, typically to all or a part of the user equipments in a cell. When using these types of channels, each data block that is sent on the channel needs to contain some kind of identity of the source and destination user equipment. Due to their limited capacity these channels are preferably used for the transmission of small portions of packet data. Another characteristic of the common channel is that the power on the uplink is controlled in a slow way and fixed for the downlink common channels. Examples of common channels in WCDMA for the uplink are the Physical Random Access Channel (PRACH) and the Physical Common Packet Channel (PCPCH). An example of a common channel on the downlink for a WCDMA-system is the Secondary Common Control Physical Channel (SCCPCH).
A dedicated channel is assigned exclusively to one user equipment and may be used by only this user equipment. This channel provides fast power control and soft handover. Dedicated channels are associated with a maximum bit rate, which is assigned when the channel is established. The transmitter may then use different sizes of transmitted blocks up to an allowed maximum value in order to accomplish a variable bit-rate channel but the assigned codes can not be used by other users when there is less to transmit than the maximum rate. An example of a dedicated channel in a WCDMA-system is the Dedicated Physical Channel (DPCH), which is a bi-directional channel.
A shared channel is shared by a set of user equipments in a cell. However, instead of identifying each transmitted block with the mobile identity, as done for common channels, the information on which blocks are used by which user equipment is provided separately, i.e. such as done on an associated DPCH to each user equipment. A scheduling function decides for which user equipment the data is sent. These channels provide a variable bitrate for each user but still uses the codes efficiently. Examples of shared channels in WCDMA-systems are the Physical Downlink Shared Channel (PDSCH) and the Physical Uplink Shared Channel (PUSCH).
Each type of channel provides a unique set of characteristics and, regarding the fact that a variety of services must be provided to the user equipments, it becomes obvious that there is not one single channel type that is optimal at every point in time and for each type of service. Therefore, it is necessary that there are control functions to select which channel type or types should be used by each user equipment at each point in time.
In WCDMA-systems, a prior known solution for this requirement is the introduction of a function for traffic volume measurement that is implemented in the Radio Network Controller (RNC) as well as in the Medium Access Control (MAC) sublayer of the user equipment. The MAC performs a scheduling of data transmission on the radio channel. For these reasons, the MAC continuously polls the data buffers in the Radio Link Control (RLC) sublayer for any data to transmit. When there is data in the buffers, the MAC takes data from the RLC-buffers and schedules the transmission of the data on the radio channel. The traffic volume measurement function in the MAC checks the amount of data in the RLC-buffers. When the total amount of data in the RLC-buffers, e.g., exceeds a limit controlled by the network, a measurement report message, which includes inter alia information about the amount of data in the buffers, is sent to the network. The network on the other hand monitors the traffic volume in a similar way, but the measurement report message is generated internally in the Radio Network Controller. The results from the traffic volume measurement function in the RNC and the received measurement reports from the mobile station are fed into a “channel type switching function” in the RNC. This function may order the connection to a mobile station to switch from a certain combination of channel types, e.g. the PRACH and the SCCPCH, to another combination, e.g. the DPCH.
WO 99/66748 discloses a method and apparatus for dynamically adapting a connection state in a mobile communication system. A packet data connection between a mobile station and a radio access network is established where the state of the connection is used to specify one of plural different types of radio channels. The connection is dynamically adapted to an optimal state based on one or more conditions relating to the connection.
FIG. 1a shows an overview of a layered protocol structure of units in a communication system as presented in FIG. 1b. The description of methods for channel switching according to the state of the art and according the present invention will rely upon these figures. This communication system consists of a mobile radio communication system 10 and a packet-switched network 20, e.g. the Internet. The radio communication system 10 is intended to provide services to a plurality of user equipments 11 that roam within its coverage area. Said user equipments 11 are connected to one or more radio base station 12, which are controlled by Radio Network Controllers (RNC) 13. The Radio Network Controller 13 is responsible for a variety of tasks related to the handling of communication traffic and system maintenance and provides also connections 14 to other networks, e.g. a packet-based network 20 that consists of a plurality of interconnected units 21,22 such that user equipments 11 in the radio communication network 10 can retrieve information from a remote unit 22 in said packet data network 20. The user equipment 11 performs an application protocol 117 on top of a layered protocol structure that uses, e.g., TCP (Transmission Control Protocol) 116 or another appropriate protocol on top of IP (Internet Protocol) 115. These protocols rely in turn on, e.g., PDCP (Packet Data Convergence Protocol) 114, which performs a compression of the TCP/IP-headers to reduce the packet sizes sent over the radio interface, RLC (Radio Link Control) 113, and MAC (Medium Access Control) 112. Finally, the physical layer 111 is responsible for the physical data exchange on the radio channel. The RNC 13 provides a corresponding protocol stack in order to be able to handle the forwarding of IP-packets. The RNC 13 is also capable to perform the necessary protocols on the physical layer (L1) 131 and the link layer (L2) 132 for handling IP-packets in the packet data network 20. Finally, apart from the appropriate layer 1 and layer 2 protocols and IP 223 the remote unit 22 in the packet data network to which the user equipment 11 intends to establish a connection must be equipped with TCP 226 and the application protocol 227 that corresponds to the application protocol 117 in the user equipment.
FIG. 2a shows a time flow diagram for the establishment of a connection according to the state of the art that shall be applied for information download, e.g. web browsing, from a remote unit in a packet data network to a user equipment in a radio access network. The application uses HTTP (Hypertext Transfer Protocol) to fetch webpages, which layout is described in HTML (Hypertext Markup Language), from a web server. HTTP in turn uses TCP on top of IP for data transmission. For each HTML-page, or sometimes even for each object on the HTML-page, a TCP-connection is established and then the bursts of data are transmitted from the web server to the web browser in the user equipment. This data exchange can be subdivided into three phases: In the first phase, a TCP-connection is established by means of sending a TCP-segment comprising a SYN-flag in its header field from the user equipment to the web server through the radio communication system and the packet data network. The web server then acknowledges the establishment of the TCP-connection by means of sending a TCP-segment comprising an ACK-flag it its header field back to the user equipment whereupon also the user equipment acknowledges by means of sending a TCP-segment comprising an ACK-flag. This establishment of a connection implies a delay time τ1. Then, in a next phase the content of the HTML-page is sent in TCP-segments from the web server. On its way to the user equipment they pass the mobile system where they are buffered and wait to be scheduled for transmission over the radio interface by the MAC. At a point T1 in time when the traffic volume measurement function detects that the amount of data in the RLC-buffers in the mobile system exceed a certain threshold value, it informs the “channel type switching function”, which triggers a change of the channel type. At a point T2 in time after a delay period τ3the channel switching is completed. There is thus a certain delay τ2 between the point T0 in time when the information download to the user equipment is started and the point T2 in time when an appropriate channel type is available for said information download. During said delay period τ2 the user will experience a slow response from the external server and a low data throughput.